Agreed, us self health advocates must navigate a lot of curiosities. ..... se la vi.
Since your surgeon was already in there, it is a little curious to me that they did not do something simple, like take a swab/sample from the wall of your colon so as to possibly get an idea of why it is black. Based on the basic theory of colon operation and the benefits from remedies such as IF#1, there is some logical speculation that might solve this curiosity but, be this as it may, you are healthy despite the mystery so perhaps leave speculation for another time.